The ocean is not in great shape.
Let’s set aside the fact that there is so much garbage being dumped in the ocean that its volume of plastic will outweigh fish by the year 2050; that’s a problem with a clear solution (greater accountability from manufacturers and consumers), if not a clear means of executing it. Less clear is how humans will adapt to the changes taking place in the very chemistry of the ocean – changes that are largely influenced by anthropogenic (human-made) factors.
These factors include ocean acidification, which not only threatens marine life in general but many coastal economies that rely on shellfish. About 30 percent of the carbon dioxide emitted by anthropogenic activities is swallowed up by the seas, which convert the gas into carbonic acid. This acid weakens the shells of creatures like crabs and oysters. It also dissolves limestone, reducing corals’ capacity to grow, and makes it more difficult for phytoplankton to absorb nutrients. Without coral or phytoplankton, the oceans’ biodiversity takes a major hit. But wait, there’s more.
The ocean is also steadily warming. Since the late 1960s, the heat content of ocean surface waters has risen precipitously – with 30 percent of the warming occurring below 700 meters. This is “contributing significantly to an acceleration of the warming trend,” according to a 2013 study published in Geophysical Research Letters (and consistent with an earlier study from Nuccitelli et al). Ocean warming is responsible for bleaching vast swaths of coral across the Pacific Ocean, including 93 percent of the Great Barrier Reef.
The bad news doesn’t stop there. A recent study published in the journal Global Biogeochemical Cycles suggests that portions of the world’s oceans are being de-oxygenated, effectively turning into dead zones for marine life.
Bad News, Without Exaggeration
This particular study has been making the rounds on popular news sites, but its lead author, Dr. Matthew Long of the National Center for Atmospheric Research, cautioned Planet Experts against engaging in the same hyperbole that creates apocalyptic headlines.
“The article definitely got your attention,” said Dr. Long, when I mentioned where I had first read about his work. However, he added that he and his colleagues can say with certainty that anthropogenic impacts on the environment, such as carbon emissions, are changing the ocean’s makeup.
In the study, Finding forced trends in oceanic oxygen, calculations were made using Earth system models in the context of natural variability (natural shifts in ocean hypoxic events). According to the authors, widespread detection of this deoxygenation may occur as soon as 2030. In the meantime, the study claims that dead zones should already be evident in the southern Indian Ocean and parts of the eastern tropical Pacific and Atlantic basins.
“Human-driven climate warming is driving substantial declines in the oxygen in the ocean,” said Long. “There are some regions where oxygen is totally depleted in the present day, and those regions will expand. So there will be a bigger area, very likely, in the equatorial Pacific, where there are near-zero oxygen concentrations.”
This does not mean that the oceans will be completely oxygen-less in 15 years. It means there are hypoxic pockets in the ocean now and they will increase as long as the planet’s carbon saturation grows and the ocean’s temperature rises.
Long’s findings correlate with a study published in late 2014 in the journal Global Change Biology, which predicted the number and size of ocean dead zones will increase as the globe warms. In fact, said Long, this is a phenomenon that has been recognized by the scientific community for well over a decade. “To be honest, I’ve been surprised by how much attention this paper has gotten,” he admitted.
“Our contribution here is not to describe a new phenomenon that was unrecognized,” he added. “Really, what we did was approach the problem with a new technique that allowed us to fairly accurately – in the context of a model – quantify the range of natural variation in dissolved oxygen over broad segments of the ocean and make a direct comparison of the human warming-caused trends in dissolved oxygen to that envelope of natural variability.”
In other words, Long and his team focused on quantifying when the “forced trend,” or human-driven climate factors, would exceed the natural shifts in ocean oxygen content.
A Vicious Cycle
On land, global warming can raise temperatures and trigger droughts, but the impact on ocean ecosystems is even more intense. Rising atmospheric temperatures affect the oxygen content in both the surface and interior waters of the ocean by wreaking havoc on the tiniest of living things.
More than half the Earth’s oxygen is produced by phytoplankton (also known as microalgae) that feed on the sun’s light. When the ocean surface (highly dependent on the temperature of the air above it) heats up, phytoplankton die, reducing the soluble oxygen in the immediate area. Masses of dead phytoplankton also deoxygenize deeper waters when they sink and dissolve the oxygen through the process of decomposition.
In a healthy marine ecosystem, oxygenized surface waters circulate down to replenish the ocean interior. But warm water, in addition to being lethal to phytoplankton, is also more buoyant, which means the rate of exchange between surface and depth declines, starving the interior of fresh oxygen and stranding phytoplankton in warmer, inhospitable temperatures.
The last two years have set unprecedented records in global temperature increases and 2016 is set to be just as hot or hotter. Fifteen of the 16 hottest years on record have all occurred in the 21st century, and the last time the planet experienced a record cold year was in 1909. As humans continue to pump carbon and other greenhouse gases into the atmosphere, there’s a sad but significant likelihood that oxygen will continue to be sucked out of the ocean for the foreseeable future.
“In terms of forestalling ocean deoxygenation I don’t really see a lot of options other than to mitigate CO2 emissions and reverse the warming trend,” said Dr. Long. “It’s intrinsically linked.”
Dr. Michael Mann, Planet Expert and Distinguished Professor of Meteorology at Penn State University, offered this reflection on Dr. Long’s study:
“This latest study adds one more item to the list of insults we are inflicting on the oceans through our continued burning of fossil fuels. Ocean life and marine ecosystems must now simultaneously contend with the triple threat of warming waters, increased acidity, and now—we’re learning—lower oxygen levels. Any one of these challenges alone would be daunting.
“Just a week after learning that 93 percent of the Great Barrier Reef has experienced bleaching in response to the unprecedented current warmth of the oceans, we have yet another reason to be gravely concerned about the health of our oceans, and yet another reason to prioritize the rapid decarbonization of our economy.”